An electrophotographic photoreceptor normally comprises a photosensitive layer provided on an electrically-conductive support. Examples of the photosensitive layer structure include single-layer type photosensitive layer and laminated type photosensitive layer comprising a plurality of functionally separated layers, i.e., charge-generating layer and charge-transporting layer.
An electrophotographic photoreceptor is normally expected to meet all the following requirements for properties:
(1) To be capable of being charged to a proper potential in the dark place; PA1 (2) To be sufficiently capable of retaining electric charge in the dark place; PA1 (3) To be photosensitive enough to release electric charge rapidly and sufficiently when irradiated with light; PA1 (4) To be capable of forming an electrophotographic photoreceptor having a proper area; PA1 (5) To have an excellent repetition stability; PA1 (6) To have an excellent durability; PA1 (7) To be inexpensive; and PA1 (8) To be harmless to human being.
An electrophotographic photoreceptor comprising an organic photoconductor (OPC) is normally of the laminated type, because such photoreceptor can easily be functionally controlled to meet the foregoing requirements.
The function of a charge-generating layer is to generate electric charge when irradiated with light in an electric field. A charge-generating layer normally comprises a charge-generating material dispersed in a binder resin. It is an important layer which determines the photosensitivity of an electrophotographic photoreceptor.
The photosensitivity of an electrophotographic photoreceptor depends on the efficiency at which the charge-generating material absorbs light to generate electric charge (efficiency of generating electric charge) and the amount of the charge-generating material.
In recent years, various printers and duplicating machines using such electrophotographic system have been marketed. Thus, it has been desired that various properties, particularly the photosensitivity of the electrophotographic photoreceptor be optimized to meet the requirements for various types of machines.
Accordingly, it is necessary that the photosensitivity of the electrophotographic photoreceptor be adjusted for every type of machine to meet these requirements.
In order to adjust the photosensitivity of the electrophotographic photoreceptor, it is necessary that a charge-generating layer which shows a proper photosensitivity corresponding to the type of machine in which it is used be formed on an electrically-conductive support.
One of methods for adjusting the photosensitivity of the electrophotographic photoreceptor is to adjust the thickness of the charge-generating layer.
In the case where the dip coating method, which is an ordinary process for the preparation of an electrophotographic photoreceptor, is used, if the thickness of the charge-generating layer is reduced in an attempt to adjust the photosensitivity of the electrophotographic photoreceptor, the thickness of the resulting electrophotographic photoreceptor is drastically uneven, adding to the dispersion of the photosensitivity of the electrophotographic photoreceptor.
Another method for adjusting the photosensitivity of the electrophotographic photoreceptor is to use a plurality of materials having different efficiencies of charge generation as charge-generating materials to be incorporated in the charge-generating layer.
The foregoing technique involving the use of a plurality of materials have heretofore been extensively studied. Examples of the technique for adjusting the photosensitivity of the electrophotographic photoreceptor with a plurality of materials will be given below. (1) JP-A-2-280169 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that the use of a mixture of (1) a phthalocyanine selected from the group consisting of copper phthalocyanine, aluminum phthalocyanine, indium phthalocyanine and vanadium phthalocyanine, (2) titanyl phthalocyanine and optionally (3) a phthalocyanine selected from the group consisting of aluminum phthalocyanine and metal-free phthalocyanine as a charge-generating material makes it possible to obtain a functionally-separated electrophotographic photoreceptor which can be adjusted for photosensitivity.
In Example 1 of the above cited patent, a metal-free phthalocyanine (H.sub.2 Pc) and aluminum phthalocyanine (AlClPc) were used. Dispersions comprising the metal-free phthalocyanine in an amount of 0, 20, 40, 60, 80 and 100% by weight based on the total 10 parts of the phthalocyanine compounds were used to form charge-generating layers. It is disclosed that the photosensitivity of the electrophotographic photoreceptor can be adjusted with the half-decay exposure E.sub.50 varying within a range of from 4 to 6 erg/cm.sup.2 while the proportion of the metal-free phthalocyanine is within a range of from 0 to 40% by weight.
In Example 2 of the above cited patent, indium phthalocyanine (InClPc) was used in an amount of 0, 20, 40, 60, 80 and 100% by weight based on the total weight of indium phthalocyanine and titanyl phthalocyanine (TiOPc). These combinations were each vacuum-deposited onto a support under a pressure of 2.times.10.sup.-6 torr. The deposited support was then dipped in ethanol. Another batch of the deposited support was not treated. Thus, a charge-generating layer was formed. It is disclosed that the photosensitivity of the electrophotographic photoreceptor can be adjusted with the half-decay exposure E.sub.50 varying within a range of from 2 to 6 erg/cm.sup.2 (treated with ethanol) or from 4 to 6 erg/cm.sup.2 (untreated) while the proportion of indium phthalocyanine is within a range of from 0 to 40% by weight.
In Example 3 of the above cited patent, copper phthalocyanine (CuPc) and titanyl phthalocyanine were used. Copper phthalocyanine was used in an amount of 0, 20, 40, 60, 80 and 100% by weight based on the total weight of these phthalocyanines. These combinations were each vacuum-deposited onto a support under a pressure of 2.times.10.sup.-6 torr to form a charge-generating layer. It is disclosed that the photosensitivity of the electrophotographic photoreceptor can be adjusted with the half-decay exposure E.sub.50 varying within a range of from 4 to 8 erg/cm.sup.2 while the proportion of copper phthalocyanine is within a range of from 0 to 40% by weight. (2) JP-A-5-241361 (which corresponds to EP548809 A1) discloses that the provision of a photosensitive layer comprising oxytitanium phthalocyanine and dihalogenotin phthalocyanine makes it possible to adjust the photosensitivity of the electrophotographic photoreceptor.
The above cited patent discloses in its examples that the use of a dispersion comprising oxytitanium phthalocyanine and dichlorotin phthalocyanine at a weight ratio of from 10:90 to 80:20 makes it possible to adjust the photosensitivity of the electrophotographic photoreceptor with the half-decay exposure E.sub.1/2 varying within a range of from 0.16 to 0.36 lux.multidot.sec. as measured by an electrostatic paper analyzer.
In the dip coating method, which is an ordinary process for the preparation of an electrophotographic photoreceptor, however, the range of adjustment of photosensitivity made by the dispersion comprising a metal-free phthalocyanine and aluminum phthalocyanine is 1.5 times as shown in Example 1 of the above cited patent (1). Similarly, the range of adjustment of photosensitivity made by the dispersion of the above cited patent (2) is twice.